1. Field of the Invention
This invention relates to the field of electron optics, and in particular to electron beam testing of large substrates such as flat panel display substrates.
2. Description of the Related Art
The use of electron beams to inspect and electrically test flat panel display substrates is an established technique. The different testing strategies may be characterized by the method of obtaining the test signal from each pixel in the display: mechanical probe testing; electron-beam probe testing; and voltage imaging.
Mechanical probe testing of a flat panel display substrate is illustrated in FIG. 1. During manufacture, all the signal lines 110 on the display substrate are connected together to one or more signal line shorting bars 106. Similarly, all the gate lines 112 are connected together to one or more gate line shorting bars 108. To connect the mechanical probe testing system to shorting bar 106, a mechanical probe 105 physically contacts the signal line shorting bar 106 and also is connected to the system ground, as shown. A second mechanical probe 107 connects to the gate line shorting bar 108 and is also connected to voltage supply 114. Pixel electrode 103 is connected to the control transistor 111 by line 109 and to an electrometer 104 by means of support arm 101 and a mechanical probe 102. The voltage on the pixel electrode 103 is measured by the electrometer 104. The physical probe 102 supplies a testing current to the pixel electrode 103 and hence to the control transistor 111. Testing of the pixel electrode 103 and control transistor 111 is then performed by monitoring the electrical response to the charging current using the electrometer 104. Capacitor 113 is formed by the overlap between the gate electrode of the control transistor 111 and the pixel electrode 103. Typical measurements made include the following: absence/presence of shorting between neighboring pixel electrodes, breaks or shorts in the connections to the control transistor 111, excessive leakage currents due to too-low isolation resistance in the pixel electrode 103. The signature of a properly-functioning pixel drive circuit is characterized, as well as the signatures resulting from various pixel malfunctions, such as shorted or open lines, degraded insulating regions within the pixel element, neighboring pixel elements shorted together, etc. Thus mechanical probe testing allows identification of various pixel defects.
Electron-beam probe testing is illustrated in FIG. 2 and is similar to mechanical probe testing, described above, except the third mechanical probe 102 (shown in FIG. 1) has been replaced by an electron beam 120 which supplies the charging current to the pixel element 103. Secondary electrons 121 emitted from the pixel electrode 103 are collected by detector 122 to form a voltage contrast signal (similar to the signal generated by electrometer 104 in FIG. 1). The many advantages of electron-beam probe testing over mechanical probe testing are: no-contact and thus no risk of contact damage; faster selection between pixel elements to test; and the opportunity for fast rechecking of all pixels failing a first-pass testing procedure. The electron beam 120 is generated by an electron optical column; there are some examples in the prior art of testing systems with multiple columns (typically 2-4), each column producing a single electron beam.
Voltage imaging is illustrated in FIG. 3, where all the pixels in display substrate 150 are being inspected in parallel. Light 156 from light source 155 illuminates a splitter mirror 153. Reflected light 157 off the upper surface of the splitter mirror 153 illuminates the under surface of an electro-optic modulator 152. An optional interface card 151 may be interposed between the display substrate 150 and the upper surface of the electro-optic modulator 152 to improve the coupling of the voltages on the display substrate 150 to the electro-optic modulator 152. Due to the electro-optic interaction of the voltages on the display substrate 150 with the modulator 152, the reflectivity of light 157 off the lower surface of the modulator 152 is affected. Light 158 represents that fraction of light 157 which is not reflected off the splitter 153, instead passing downwards through the beam splitter 153 to be collected by a CCD camera 154, which is coupled to display electronics (not shown) to generate a voltage image of the display substrate 150. A significant disadvantage of this method is the need to fabricate a new modulator 152 for each new display design.
All three of the flat panel display substrate testing systems and methods described above suffer from throughput limitations which will only get worse as the size of display substrates continues to increase. There is a need for flat panel display substrate testing systems and methods that have higher throughput and that are more readily scalable to larger substrates.